The present invention relates to the device and method for fixing a developer, and in particular, to the device and method for fixing a developer which are useful for an electrophotographic process.
As an apparatus making use of an electrophotographic process, such as a copying machine, a facsimile terminal equipment, a printer, etc., there are generally known various types such as a diazo type, an EF type, a PPC (Plain Paper Copier), etc. Among these types of apparatus, the PPC is currently most extensively employed. This PPC is constituted, as a basic structure, by various members for carrying out an electrification step, an exposing step, a developing step, a transferring step, and a fixing step, as well as an additional member for carrying out a cleaning step.
Namely, according to this PPC, the electrophotographic process thereof comprises a step of electrification wherein a photosensitive body is entirely exposed to a corona discharge so as to uniformly generate an electric charge (ion) on the surface of the photosensitive body; a step of exposure wherein a reflected light of a light source such as a halogen lamp is projected through an optical system comprising lenses and etc. onto the surface of the photosensitive body to form a shaded image thereon, thereby leaving the electric charge in the region where the reflected light is not irradiated so as to form an electrostatic latent image thereon; a step of development wherein a developer (toner) is electrically attracted to the electric charge on the surface of the photosensitive body thereby performing the development of the electrostatic latent image; a step of transferring wherein the surface of the photosensitive body bearing the toner image thereon is caused to contact with the surface of recording material (recording paper) and a corona discharge is again applied to the back surface of the recording paper thereby causing the toner to transfer to the recording paper, a cleaning step wherein the toner that remained on the surface of the photosensitive body is wiped off, thereby making the photosensitive body to be repeatedly usable, a fixing step wherein the toner adhered onto the recording paper through a weak electrostatic attractive force is caused to melt, thereby fixing the toner to the recording paper.
In this fixing step, the system which is currently most extensively employed therefor is a heat roller fixing system. According to this heat roller fixing system, a heating energy is supplied from a pair of heated and press-contacted rollers each being formed of a hollow metallic roller which is provided at the center thereof with a heater to the recording paper bearing an unfixed toner image and passing through this pair of heated rollers, thereby fixing the toner image to the recording paper (a two-way heat fixing method) (for example, Japanese Patent Unexamined Publication H5-188807).
However, there is a problem in this heat roller fixing system employing the two-way heat fixing method that comprises a so-called high temperature off-set phenomenon where the melted toner is caused to adhere onto the fixing roller which is disposed on the image-fixing side is more likely to be generated.
In the meantime, since a color toner requires a larger quantity of thermal energy for this fixing as compared with a monochrome toner and at the same time, requires a higher grossness to the recording paper as well as a higher transmittance to the OHP as compared with a monochrome toner, the color toner is generally constituted by a resin which is more excellent in so-called sharp melt as compared with the monochrome toner. Therefore, since the aforementioned two-way heat fixing method is generally employed, and moreover, since the segmentation of toner tends to be generated due to a decrease in cohesive force between the toner particles in the electrophotographic apparatus which makes use of a color toner, the high temperature off-set phenomenon is caused to be generated more prominently.
Under the circumstances, it becomes absolutely necessary, in the case of the conventional fixing device of electrophotographic apparatus making use of a color toner, to coat an off-set preventive agent such as a silicone oil on the surface of the roller in order to minimize the surface energy.
In this case, the coating and supply of an off-set preventive agent are generally performed by coating means such as a coating roller which is designed to be contacted with a fixing roller. However, this coating means is accompanied with various problems such as (1) the mechanism for uniformly coating the oil is complicated, thus inviting an increase in the manufacturing cost of the fixing device; (2) Once the oil is spilled out of the fixing device, it will badly affect the other steps (such as development and transferring) of the electrophotographic apparatus; (3) while the image printed by the electrophotographic apparatus is demanded to be higher in grossness, an increase in coating of the oil will most likely give rise to a deterioration of transmittance to the OHP and to the generation of image defects such as the generation of oil lines; (4) A periodical maintenance of the supply of oil for instance is required so that the aforementioned coating means is not user-friendly.
Therefore, it is now desired to develop a device and method for fixing the color toner, which make it possible to enlarge a region in which not only a sufficient strength of fixing can be realized irrespective of the specifications of recording paper (a thick paper as well as a thin paper), but also the off-set phenomenon can be prevented to occur (i.e. a fixable region).
On the other hand, an electrophotographic apparatus which makes use of a monochrome toner is frequently subjected to a long period of continuous fixing operation such as a multi-printing, etc. In this case, the heat of the fixing roller is transmitted to the pressing roller, thereby minimizing the difference in temperature between the fixing roller and the pressing roller, thus causing the high temperature off-set phenomenon to tend to be generated. In recent years, in order to overcome this problem, measurements have been taken to add a wax to a toner so as to minimize the high temperature off-set phenomenon, and at the same time, a fixing apparatus which makes it possible to realize an oil-less operation has been put to practical use. However, there are still problems that (1) when the releasability of the covering layer formed on the surface of the fixing roller is deteriorated due to a long period of use, the fixable region is caused to become narrow, thereby giving rise to the generation of the off-set phenomenon especially when a thin recording paper is employed; (2) when a recording paper which is smaller in width than that of the fixing roller is continuously fed to the fixing roller, the portion of the fixing roller which is not contacted with the recording paper is caused to rise in temperature, so that when a recording paper of the ordinary size is subsequently fed to the fixing roller, the high temperature off-set phenomenon is caused to generate, and (3) when a wax is added to a toner, the mechanical strength of the toner resin is caused to become lower, or the wax may be caused to melt at the occasion of kneading a binary developer consisting of the toner and a carrier, thereby deteriorating the dispersion of toner, etc. Therefore, it is now desired to develop an apparatus and method for fixing a monochrome toner, which is capable of minimizing the generation of the high temperature off-set phenomenon without necessitating the addition of a wax to the toner.
Further, the reduction of power consumption becomes an important problem in recent years in view of protecting the environment, so that it now becomes imperative to minimize the power consumption at the fixing device portion where the power consumption is the highest in the electrophotographic apparatus. However, according to the heat roller fixing system where the two-way heat fixing is employed, the magnitude of thermal energy to be consumed for the recording paper, in addition to that to be consumed for the toner, is relatively large, and it is impossible to reduce the power consumption in the aforementioned conventional heat roller fixing system.
Under the circumstances, Japanese Patent Unexamined Publication H7-334023 describes a fixing device wherein the temperature of a pressing roller is controlled to alter based on the measured value of the magnitude of power fed to a recording paper from a fixing roller immediately before the recording paper is transferred to the fixing device, thereby making it unnecessary to detect the temperature of the pressing roller. However, this fixing device disclosed therein also adopts the heat roller fixing system where the two-way heat fixing is employed. Namely, according to this fixing device, a one-way heat fixing system where a thermal energy is supplied only through the developer layer side is not taken into account at all.